1. Field of the Invention
The present invention relates to systems for measuring the resistivity of fluids, and more particularly, to an eddy-current type probe for downhole measurement of drilling fluid resistivity and variations therein. The invention is particularly suited for measurement while drilling applications.
2. Description of the Background
In the drilling of boreholes for oil and gas wells, it is customary that the borehole be filled with a supply of drilled mud to maintain pressure control over the various earth formations through which the borehole passes. During the drilling operation drilling mud is continuously circulated between the surface and the bottom of the borehole in order to cool the drill bit and to carry away cuttings as they are removed from the earth formations by the drill bit. Drilling mud is pushed down the axial opening in the drill pipe, out openings in the drill bit and back up the borehole towards the surface through the annular space between the outside of the drill string and the walls of the borehole.
As huge quantities of drilling mud pass the drilling bit, the chemical and physical properties of the drilling mud are often changed. For example, should the drilling bit be penetrating an earth formation which contains a substantial quantity of salt water, an influx of such connate fluids into the borehole will produce a corresponding increase in the salinity of the drilling mud and a corresponding decrease in the electrical resistivity of that mud. Further, as the drill bit penetrates a fluid permeable formation having little or no connate water contained therein, water from the mud will be carried into the formation and the solid particles carried in suspension within the drilling mud will be caked onto the walls of the formation. As a result of this phenomenon there will be a net loss of chloride ions from the associated portions of the drilling mud which will cause a corresponding increase in the electrical resistivity of the mud due to a decrease in its salinity.
It is noted that there is very little net change in the resistivity of the drilling fluid except that which occurs very near the drilling bit. Thus, measurement or continuous monitoring of the resistivity of the drilling fluids at or near the drilling bit will produce data which is indicative of characteristics of the formation through which the drilling bit is at that time passing.
Prior systems have generally made drilling mud resistivity measurements only at the surface. Further, prior techniques have made comparative measurements wherein the differences between the resistivity of the drilling fluids flowing out of the borehole are correlated in an effort to log the characteristics of the formations through which the drilling bit has passed. It is to be noted, however, that if measurements are only made at the surface, e.g., at the returning drilling fluids reservoir, there will have been a substantial time delay between the passage of the drilling bit through the formation and the measurement of the changed resistivity of the drilling mud associated with its passage through that particular formation. Thus, in order to enable the use of drilling fluid resistivity measurements to provide an accurate, real time indication of the physical characteristics of the formation through which the bit is passing during the drilling operation, the measurements must be made at or near the drilling bit and instantly monitored therefrom. This is a very difficult undertaking for most of the prior techniques of drilling fluids resistivity measurements.
Most prior drilling fluid resistivity monitoring systems for use down hole have employed either a two electrode configuration or a four electrode configuration. In the two electrode systems, current is induced to flow between two spaced apart electrodes between which is positioned the fluid, the resistivity of which is to be measured. Although the current flow between the two electrodes is indicative of the resistivity of the fluid therebetween, it is noted that this same current flow produces a polarization of those electrodes and thereby inhibits an accurate measurement of the resistivity of the fluid.
In the four electrode system, two electrodes are used for stimulation of electric current in the fluid medium and two electrodes are used mainly for monitoring the potential difference across the fluid medium. That is, the measuring electrodes draw no current and, therefore, there is no polarization inherent in their operation as in the case of two electrode systems. The principle problem inherent in a four electrode system is the difficulty of measurement of the resistance of very small volumes of fluids with the necessary accuracy. For example, in a measuring while drilling application of the present invention, mud resistivity variation will often occur only in a very narrow volume of space positioned between the outside of the drill collar and the wall of the formation, that is, within a distance typically on the order of three quarters of an inch. Resistivity measurement of such small fluid volumes is beyond the capability of prior four electrode systems.
The system of the present invention overcomes many of the disadvantages of the prior systems and provides for the rapid measurement of fluid resistivity at a location near the drilling bit. The system thus facilitates the delivery to the surface of highly accurate, relatively noise-free, real-time signals indicative of the character of the formation being penetrated by the bit. Such real-time information regarding the nature of the formation being penetrated allows the driller to responsively alter the characteristic of the mud when required.